Kelvins - the standard units for measuring Color Temperature (more precisely Blue vs. Orange color cast) - are not a linear scale.
- if you are taking photographs under low-Kelvin-value lights (which look orange or yellow): every small Kelvin change will cause visible change in the photograph tonality
- if you are taking photographs under high-Kelvin-value (very blue lights or shadows): Kelvin change needs to be much bigger in order to generate any visible tonality change
To put it differently, even though the discrepancy between your Lumu Power and your other color meter (Sekonic, Minolta, etc.) is significant, your Lumu Power is most likely still giving you correct results.
When the Kelvin values are very high (10,000+K) the difference between various color meters can be up to 3000K and your final photo/video/light will still have a perfect White Balance. On the other hand, when you're working in low-Kelvin conditions (around 2800K), the difference can be around 300K.
Lumu Power's strength lies in metering low-Kelvin values, so in strong light, which are most common. We are, however, planning to improve high-Kelvin measurements in one of our future app or firmware upgrades.
You can read more about Color Temperature and Lumu Power here.
If you believe that you received a faulty unit that doesn't measure Color Temperature correctly, please read this troubleshooting article or contact us at firstname.lastname@example.org.